Question

In a nuclear reactor, heavy water is used to:
A. provide high speed to neutrons
B. reduce the speed of neutrons
C. capture neutrons produced by nuclear fission
D. transfer the heat from the nuclear reactor

Answer 1

Hint: A nuclear reactor is based upon controlled nuclear chain reaction. It consists of various components such as coolant, fuel, moderator, control rods. Here in this question, we are asked for what purpose heavy water is used. Heavy water is used as a moderator in the reactor. Let us see what functions the heavy water performs being a moderator.

Complete step-by-step answer:We know that the nuclear reactor is based on the principle of controlled nuclear chain reaction. In nuclear chain reaction a nuclear fission produces neutrons, which triggers another fission and soon a chain of fission is formed.
It was observed that in a fission, neutrons liberated in fission unless slowed down will escape from the reactor without reacting with the radioactive nuclei. Also, it was found that slow neutrons have a much higher intrinsic probability of inducing fission of the radioactive nuclei than the fast neutrons. Therefore, to slow down the neutrons or to reduce the speed of neutrons the elastic scattering of fast neutrons with light nuclei is used. Light nuclei in a reactor are called moderators. Heavy water $\left( {{D}_{2}}O \right)$is commonly used moderator.
Therefore, heavy water is used as a moderator which slows down the speed of neutrons. Also, one should remember that moderators should not capture the neutrons. Also, it is used to transfer heat from nuclear reactors.

Hence, option B and D are the correct options.

Note:Heavy water is commonly used as a moderator in the reactor which uses uranium as a fuel, since it has less absorption probability of neutrons than water. Also, heavy water is light in weight which makes it a perfect moderator.